This invention relates to manufacturing techniques for so called "wound film capacitors".
This type of capacitor is well-known and is routinely manufactured in large quantities. In these capacitors, the plates consist of thin metal films or foils, separated by a plastic film dielectric. The capacitors are referred to as "wound" because the metal-dielectric-metal sandwich is wound into a spiral laminate, thereby providing high values of capacitance in a relatively compact format.
For subsequent use, the spiral, which is initially round in its cross-section, is flattened to give it a race track-like cross-section. Connecting leads are then attached to the opposite capacitor plates as follows. The metal layers forming the capacitor plates are so positioned that one plate extends all the way out, or even slightly beyond one end face of the flattened spiral laminate, while the other plate extends to or slightly beyond the opposite end face. Metal, sometimes referred to as "schoopage" metal, is then applied to each end face of the laminate by spraying it on (in molten form), thereby making contact with all the turns of one capacitor plate at one of the end faces and with all the turns of the other capacitor plate at the opposite end face of the spiral laminate. Subsequently, wire connecting leads are welded to these sprayed-on metal contacts.
The resulting assembly is encapsulated, with the connecting wires protruding for connection to other electronic circuit components.
A conventional technique for carrying out the manufacturing operations for wound-film capacitors, including the specific steps outlined above, is by means of high-speed, semi-automated machinery.
For the spraying-on of the schoopage metal, there is formed a continuous web consisting of two paper tapes, between which are positioned, in a row, the individual flattened metal-and-dielectric spiral laminates. These are oriented so that the flat surfaces of the spiral are in contact with the respective tapes, while the rounded ends of adjacent units are positioned facing each other in the longitudinal direction of the tapes. The end faces of the spirals are adjacent to the side edges of the tapes. Between any two consecutive spirals, the tapes are pinched together, thereby effectively forming spaced-apart sleeves in which the consecutive laminates are held. Thus, there is formed a configuration which, in its geometry, can be roughly analogized to an ammunition belt, with the individual laminates corresponding to the bullets in the belt.
The web which is formed in this way is then subjected to metal spray from opposite sides of the web. The intention is to have this sprayed-on metal reach only the end faces of the laminates in order to form the desired electrical contacts to the respective capacitor plates exposed at these end faces. In doing so, the tapes are intended to serve as masking means, for keeping the metal spray confined to these end faces. However, the previously described configuration of laminate-holding sleeves, spaced apart by the pinched-together portions of the paper tapes, leaves roughly triangular gaps between these tapes, adjacent to the rounded ends of the spiral laminates. This, in turn, permits sprayed-on metal to also unintentionally reach and become deposited on the above-mentioned rounded ends of the laminates. The material so deposited is sometimes referred to as "overspray".
This is a highly undesirable phenomenon, because the presence of such overspray can adversely affect the quality of the capacitor. In some cases, the metal overspray can actually provide a complete metal path between opposite end faces of the laminate, thereby in effect bridging the capacitor plates and shorting-out the capacitor completely. Even if this metal bridging path is not complete, it can still substantially reduce the internal resistance of the capacitor, which may render it ineffective for its intended purpose.
Consideration has been given to applying a material, such as Teflon, to the surface of the laminates, in order to prevent the metal overspray from sticking to this surface, but this has not proven to be effective.